For high-bit-rate, long-haul fiber-optic communications p-doped/intrinsic/n-doped (PIN) photodiodes and avalanche photodiodes (APDs) are frequently used as photodetectors due to their high sensitivity and bandwidth. Planar and mesa structures are two commonly used configurations for PIN Photodiodes and APDs. Mesa structure PIN photodiodes and APDs are sometimes grown by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD). These fabrication techniques allow the thickness of the layers and the wafer to be accurately controlled.
Referring to FIG. 1, a mesa structure PIN 2 known in the prior art is shown. The structure includes a top metal contact 8, two bottoms metal contacts 12, a p-doped Indium Gallium Arsenide (InGaAs) ohmic contact layer 64 lattice matched to Indium Phosphide (InP), a p-doped InP layer 68, an intrinsic narrow bandgap InGaAs absorption layer 76 lattice matched to InP, a n-doped InP layer 80, and passivation regions 32.
In fabrication after the layers 80, 76, 68, 64 are sequentially deposited, the mesa structure 84 is formed by chemical etching through the p-doped layers 64, 68 and the intrinsic absorption layer 76. Next, the exposed sidewalls of the p-doped layers 64, 68 and the intrinsic absorption layer 76 that define the mesa structure 84 are passivated with dielectric materials, such as SiO2 or SiNx. As part of this process, defects are inevitably introduced into the p-doped layers 64, 68 and the intrinsic absorption layer 76. The intrinsic InGaAs absorption layer 76 has a low bandgap and the mesa etching introduced defects create extra intraband energy levels. These in turn lead to a high dark current. The dark current in InGaAs PIN photodiodes and APDs fabricated according to the above method is one factor in the generally low reliability of these devices. The low reliability of these devices includes low sensitivity and high noise. These disadvantages significantly restrict the use of InGaAs PIN photodiodes and APDs in optical communications systems.
Referring to FIG. 2, a planar structure PIN photodiode 4 known in the prior art is shown. The structure 4 includes a top metal contact 8, two bottom metal contacts 12, an intrinsic InGaAs layer 16, an intrinsic InP layer 20, an intrinsic absorption InGaAs layer 76, a n-doped InP layer 28, passivation regions 32, a p-doped InGaAs diffusion region 36, and a p-doped InP diffusion region 40.
During fabrication of the planar structure PIN photodiode 4, the n-doped InP layer 28, the intrinsic InGaAs layer 76, the intrinsic layer InP 20, and the intrinsic InGaAs layer 16 are sequentially deposited. The p-doped regions 36 and 40 are then formed by diffusing, for example, Zinc (Zn) or Cadmium into the top central region of the device 4. After the diffusion step, the top metal contact 8 and the passivation regions 32 are added.
Although avoiding the introduction of defects into the intrinsic InGaAs layer 76 during passivation, planar structure PIN photodiodes 4 have disadvantages in device performance and design flexibility. The introduction of the p-dopant by diffusion is not a precise process, and, therefore, the thickness of the p-doped regions 36 and 40 cannot be accurately controlled. In some instances the p-dopant diffuses into the intrinsic InGaAs layer 76. In other instances the p-dopant does not diffuse completely through the intrinsic InP layer 20, or even through the intrinsic InGaAs layer 76. Another disadvantage of planar structure PIN photodiodes 4 is their higher parasitic capacitance. The parasitic capacitance exists between the conductive substrate and device pad. Mesa structure devices can avoid this problem, however, by employing a semi-insulating substrate.
An additional disadvantage of planar structure PIN photodiodes 4 is that their fabrication process is complex. In particular, the diffusion process requires that the surface of the layer to be doped be carefully prepared. A further disadvantage of planar structure PIN photodiodes 4 is the control of hazardous materials as part of the dopant diffusion. For example, in Zn diffusion, As, P, Zn3P2, and Zn3As2, are heated to approximately 550C. At this temperature, small evaporated and inhaled doses are lethal.
What is needed are PIN photodiodes and APDs that overcome the disadvantages of current PIN photodiodes and APDs.